Constant Current Lead Acid Battery Discharger

[sa87]

I need to design a constant current Lead acid battery dis charger.
Specifications
12V Lead Acid Battery discharger
discharge current rating 0-8A
We need to get the constant current from the battery through out the discharge period until terminal voltage 10.8V.
I want to know about a suitable circuit diagram and parts for this design.
Can anybody help me.....

 

[MrAl]

Hi,

Just to get the ball rolling, you might want to note that 8 amps at 12 volts is 96 watts, so your load will have to be able to handle that without a break down.

You also will want to note that you'll need a low voltage cutoff because you dont want to discharge it too far down.

One traditional way to do this is to with an op amp and transistor. To get to the 8 amps you'll need at least two transistors, one to drive the other. The bigger one has to be able to handle the current and the power and will need a heat sink.

 

[ronv]

Here is a link to a long discussion about a load tester:

https://www.electro-tech-online.com/threads/automated-load-tester.125069/

This was a much larger one and had several bells and whistles like battery fully charged indicator, 6 volt or 12 volt select, time to discharge etc.

Tell us what you are trying to accomplish. If tou just want to discharge the battery it is one thing. If you are testing them it is another.

 

[sa87]

Thanx a lot for the reply. Actually I need to perform the capacity test for the Lead acid battery. To do that dc load should be designed. I need help to design the dc load so as to discharge the lead acid battery in constant current. As the capacity of the battery differ, the dis charger should be able to change the discharging constant current.

 

[MrAl]

Hi,

What part of it do you need help with?

 

[bountyhunter]

Hi,

Just to get the ball rolling, you might want to note that 8 amps at 12 volts is 48 watts, so your load will have to be able to handle that without a break down.

Actually it's 96W, and that would take a big heatsink/fan assembly.

You would need a few FET's in parallel. Use it in a constant current loop with adjustable reference voltage.

 

[MrAl]

Hi,

Yeah ha ha, it's fixed now. Lots of power, going to need a really good heat sink. 8 amps isnt much though for the good transistors even bipolar. Using two transistors gets it down to 48 watts each.

Dont forget the low voltage cutout which is also important.

The attachment shows just the load tester part. It's in two sections so the two transistors can be mounted on two different heat sinks to divide the power up a little.

The low voltage cutout section would just be a comparator that shunts down the 0.8v voltage reference. This is just an example of what one might look like. The blue boxes represent the heat sink. Both transistors in the blue boxes can be mounted on the same heat sink, driver transistor and main power transistor Q1 and Q3 which are 2N3055A just for example.

 

[ChrisP58]

If you don't need a wide range of control, you can also add some fixed resistance is series with the transistors to dissipate some of the power.

There are also commercial Electronic Loads that you can buy if you don't want to build your own.

 

[sa87]

I need to design a battery test bench to perform capacity test for starting type lead acid batteries.
Full charged lead acid battery 12.6V and do the constant current discharge until 10.5±0.05v
According to the battery capacity discharging constant current should be able to change and these data should be able to take to computer. Timer should be included and also the voltage cutoff to stop the discharge at 10.5±0.05V. This should be automated one.
I was thinking about to do this using a switching dc load which keep the constant current through out the test when the battery voltage change. If somebody can give me a suitable switching load design to perform this it may be very helpful. Thanx

 

[sa87]

Adding something more about the project have to be done. The discharging constant current may vary from 1A to 8A. So this design must be suitable to discharge any constant current ranges from 1A to 8A.Can any body help me with this ............

 

[sa87]

thanx ronv for your given link. I got much idea about the project and there is some difference from mine. Any way I think MrAl idea is much better for my purpose. But there should be some changes. I need help to design constant current sink using that circuit. Here I need to way of automatically varying apply voltage to the OPAM positive edge. Please help me

 

[MrAl]

Hi,

What do you mean by "automatically varying the voltage to the op amp" ??

You would normally use a potentiometer there and maybe a voltage reference IC or zener diode. When you turn the pot, you can make the current go higher or lower. Is that what you need?

 

[sa87]

I was thinking to do the constant current load using LM324 Opam and MTP3055 MOSFET . I need to supply varying voltage input to an opam circuit. Voltage input should be given by the computer and I need a way to turn the given computer data into opam input. Please help.

 

[MrAl]

Hi,

Oh ok, well that requires another little circuit then.

I assume you know how to establish communications between the computer and your circuit. Your circuit may be nothing more than a small micro controller and using RS232 for the communications. In this case you would simply send the micro controller whatever signal you wanted it to get, and using the digital to analog converter in the microcontroller (uC) that would supply the input to the op amp non inverting terminal.
The digital to analog converter (DAC) could come in several forms, either built right into the uC or a separate DAC chip connected to the uC. It depends how accurate you need the signal to be which also determines in part the number bits needed.
The DAC could also be nothing more than a little PWM circuit where you set up the uC to output a PWM signal into a resistor and capacitor and the voltage input for the op amp comes from the voltage across the cap. With this kind of DAC as you increase the pulse width you get more output voltage.
The choice between what kind of DAC you can get away with using also depends on the speed of response you need from the whole circuit. If it doesnt have to be super fast the PWM DAC would suffice. If you need faster response then the DAC chip would be best.

 

[sa87]

Thanx MrAl . Actually I have little bit knowledge on uC. So can you give me uC circuit to drive the opam. thanx

 

[ChrisP58]

If you want to control it from a PC, I would suggest getting a usb data acquisition system. Google "usb data acquisition"
Find one that has an analog output. most are analog input only. Connect that output to the ref input of the op-amp.
Use the analog inputs to read voltage, current and temperature values back into the PC.

 

[MrAl]

Thanx MrAl . Actually I have little bit knowledge on uC. So can you give me uC circuit to drive the opam. thanx

Hi again,


Well that's great. The circuit is actually quite simple, but you'll need a little experience in connecting the RS232 port to the uC chip. If you dont have that experience you'll have to look it up on the web and there are plenty of sites that have circuits. I didnt want to actually design this whole system
If you have trouble though i'll try to help.

As Chris pointed out you may not want to even build this yourself but instead purchase an interface system.

So the design goals are:
1. Design RS232 to uC chip interface. If you dont need to read values you can use a one way communication type port interface. Many uC chips already have an RS232 peripheral on board.
2. Design a PWM DAC, or just use a DAC chip.
3. Build the constant current circuit.

The circuit is really nothing more than a uC chip, with the possible addition of another serial type DAC chip, say 12 bit. You have to program the uC chip to do the RS232 interface and to interface with the DAC chip.

 

[ChrisP58]

You might take a look at this to see it it would meet your need. https://www.westmountainradio.com/cba.php